1. Field of the Invention
The present disclosure relates to bioresorbable composites and more specifically to a fiber reinforced polymer composite material that is used to make bioresorbable products.
2. Related Art
Metal products have been used in fracture fixation due to their high strength. While these products perform well, there are a significant number of occurrences where these products can cause problems to the patient. In some cases the presence of the metal implant can cause irritation of the soft tissue around the implant, in severe cases this necessitates the removal of the implant. The procedure to remove the metal products exposes the patient to the risks associated with undergoing a major medical procedure and also adds to the overall cost of healing the original fracture. One potential solution to substantially reduce the need to remove fracture fixation hardware is to use bioresorbable devices to fix the fracture. However, the currently available bioresorbable materials and products do not have the required combination of initial strength and retention of this strength for suitable fracture healing to occur.
The currently marketed bioresorbable products include those products manufactured from injection molded polymers, polymer blends, and co-polymers. These products have been utilized in the areas of craniomaxilofacial implants and non-load bearing fracture fixation implants, such as pins and screws, for wrist and ankle applications and for reattaching soft tissues, such as ligaments and tendons, to bone. In addition, there are also some spinal products available that make use of the compressive properties of these polymers. Products including these materials are easy to process, but are limited by the mechanical properties of the materials. These materials have a tensile strength in the range of between about 50 MPa to about 100 MPa. Depending on the choice of polymer or co-polymer, products in this category retain the majority of their strength for less than about 12 weeks. Therefore, these materials are not suitable for fracture fixation applications beyond simple non-loaded pins and screws.
Other currently marketed bioresorbable products include self reinforced products that have improved strength due to orientation of the polymer during processing of the product. Even though these products have improved strength, their flexural strength is still only around 250 MPa. This limits the uses of this technology for fracture fixation to screws and pins.
Recently, devices have been manufactured from fiber reinforced polymer composites utilizing polyglycolic acid (PGA) fibers. These composites have a good initial strength, but suffer a rapid loss in strength due to the rapid hydrolysis of these fibers. Devices have been manufactured using PLLA fibers and PDLLA as the matrix material. Unfortunately, this matrix breaks down rapidly and results in the composites having a rapid loss in strength. Other attempts have used co-polymers containing PLLA as the reinforcing fiber, such as PLLA-co-PGA copolymers at a ratio of 82:18. However, there has been difficulty in finding a suitable polymer matrix material that can be processed into a composite without degrading or breaking this reinforcing fiber. Most recently, composites have been made where the matrix was a polymer with the same chemical composition as the fiber or where the matrix was a blend with the majority of the blend being a polymer with the same chemical composition as the matrix. These composites have an initial flexural strength of between 120 to 140 MPa, with most of this strength lost within about 12 weeks of use.
Attempts to slow down the degradation of the polymer matrix have included modifying the composition to increase the hydrophobicity of the polymer. However, this increases either the crystallinity of the polymer matrix, which is undesirable from a biological perspective, or it makes the polymer too ductile if a hydrophobic rubbery component, such as polycaprolactone (PCL), is added. Buffering materials, such as calcium carbonate, have also been added to polymers to slow degradation rates and improve the biological properties, such as osteoconductivity. However, in order to gain the beneficial effects of calcium carbonate it needs to be present at high levels, about 30% by weight of the composition. Since a fiber polymer composite contains at least 50% of fiber by volume, it would be anticipated that a calcium carbonate-containing matrix would interfere adversely with the interface between the polymer matrix and reinforcing fibers. This could result in the fiber-reinforced composite substantially weakening or even falling apart before complete healing of a fracture.
In order to make a suitable fiber-reinforced composite material, the fiber and matrix material have certain requirements. The fiber needs to have both a high initial tensile strength, and the ability to retain the majority of this strength, for the fracture to heal. To have a high initial strength, the fibers need to be highly orientated and be present at about 40% by volume of the composite. In addition, the fibers should also have some crystallinity, as this imparts stability against relaxation of the orientation in the fiber.
The matrix material also needs to be able to retain the majority of its strength for a suitable time, approximately between about 6 to about 12 weeks, for the fracture to heal. In order to accomplish this, the matrix should have a sufficiently high initial molecular weight. As the polymers degrade, the molecular weight decreases and the polymers become brittle and lose their mechanical properties. Additives, such as calcium carbonate or other buffering materials, can be added to the matrix to control the degradation rate. The amount of the buffering material should be around 30% by weight of the matrix without adversely interfering with the interface between the polymer matrix and the reinforcing fibers.
In addition, the matrix material needs to be processable at a temperature which is low enough to not significantly affect the strength of the fiber and adhere well enough to the fiber to allow stress transfer from the matrix to the fiber. To accomplish this, both semi-crystalline and amorphous co-polymers can be used. Semi-crystalline co-polymers are typically composed of lactic acid and one or more additional monomer units whose function is to lower the melting point of the co-polymer matrix to a point where the strength of the fiber is not affected during the consolidation step. Amorphous or non-crystalline materials, such as poly (D-lactide) acid polymers, are suitable for processing with the fiber, as they soften at relatively low temperatures. However, these materials do not have a long strength retention time. This strength retention can be improved by incorporating a buffering material, such as calcium carbonate, into the matrix material. In this case, the calcium carbonate acts as both a buffer and also reduces the thermal sensitivity of the polymer to breakdown during processing. Taken together, the affect of the calcium carbonate is to both slow the rate of degradation of the polymer and help preserve the molecular weight during processing, without adversely interfering with the interface between the polymer matrix and the reinforcing fibers.
The present disclosure incorporates these requirements to produce a bioresorbable material which has a high initial strength and retains a significant proportion of this strength for a useful time.